The vast majority of bicycles currently marketed for recreational bicycling by older children and adults have a multi-speed drive that includes a freewheel associated with the rear wheel and a derailleur to shift the drive chain among the sprocket wheels of the freewheel. The derailleur is attached to a hanger that is part of a plate that also includes the rear wheel axle dropout. The dropout/hanger plate is usually welded or brazed to the chainstay tube or to both the chainstay and seat stay tubes of the bicycle frame and, thus, is a permanent part of the frame.
The hanger portion of the dropout/hanger plate projects downwardly below the dropout, and the derailleur extends generally downwardly even farther. The derailleur, therefore, is quite exposed to impacts from objects on the ground, especially rocks or roadway curbs. Occasionally, the hanger portion of the dropout/hanger plate is broken off, commonly at the bolt hole where the derailleur is attached to it and which inherently presents the minimum cross-sectional area of the hanger portion and is therefore a zone of weakness, by an impact on the derailleur of an object along the path of the bicycle. The incidence of broken dropout/ hanger plates is particularly high in the case of mountain bikes, which are frequently ridden off-the-road. It is not possible, as a practical matter, to repair satisfactorily a broken dropout/hanger plate, because brazing the broken-off part to the vestigial part does not provide adequate strength. While welding is possible, the expense of preparing the pieces, making the weld, grinding it to accept the derailleur and repainting the area for appearance and protection will usually exceed the cost of a new frame. In the case of bicycles with aluminum frames, an attempt to weld the broken piece to the vestigial piece will almost certainly cause permanent thermal distortion of the chain stay and seat stay tubes of the frame adjacent the welding site and ruin the frame. Thus, a broken derailleur hanger almost always produces a bicycle frame destined for the scrap pile.